As a huge 9th Ed fan, all the changes I've seen so far sound really great to me. Obviously we know basically zero specifics so who can say how it will work out, but it sounds like GW are setting themselves (and us) up for success.
Detachments: Huge fan. Good for balance, good for accessibility. I've played hundreds of games of 9th but my incessant army hopping gives me a bit of insight into the newbie experience, and I agree strats in particular are Too Much. Detachments as described are enough moving parts to express the character of a force and maintain interest for game focused players, few enough to be learnable and referenceable. Playing with a two sheet official printout and not needing to hit Wahapedia every five minutes is a huge win.
Balance wise they are self contained, and make life a lot easier for GW, allowing more precise buffs or nerfs, or the option to just *give* a struggling faction a strong new Detachment.
My prediction for a downside is I think there'll be a lot of detachments and as long as a faction has at least one competitive one, they won't spend a lot of effort keeping the others up to date. So if there's a detachment you really like for fluff reasons but its mechanics are mediocre or bad, they'll probably stay that way while other detachments for your faction tick the balance boxes.
This reminds me of Kill Team and to some extent AoS, and both are good.
Army Composition: Looks like AOS and that's good. I've enjoyed playing with Arks detachments, and I still often take troops if they're good enough (<3<3<3 Infiltrators, and while Matt hasn't had good luck with Noise Marines I assure those jerks get a ton of play). Detachments granting Battleline will make a lot of sense, other than that hopefully they do a good job of making each datasheet appealing. It won't work for Marines, but on the other hand not having such a brutal cutoff for space in say Elites will help armies with lots of units find some table time for their less played ones.
Combat Patrol: This sounds good. The idea every CP box is balanced against every other sounds *unlikely* but making it its own game mode with fixed armies lets them do things for balance here that won't escape containment into the main game so it's plausible. I really like that there's an incredibly simple way to start play for every single faction. This has the potential to mitigate my single biggest complaint about 9e (and many many other games I've loved over the years) - excessively frontloaded learning. If they've really got it to the point where a newbie can buy and assemble one box, play a few decent games into anyone with it and then move on to Incursion making more informed decisions then it's a huge win.
If it's even half as good as Underworlds' Rivals format it will be stellar (have we mentioned how good Rivals is?)
The downside is no cheeky Combat Patrol games with our wider forces to grab quick XP for Crusade or for folks who hit Higashi Betsuin at 9am waiting for later risers
But to be honest I don't think Incursion is much slower now with practiced players so if Detachments streamline the game enough games should be brisk at that size.
USRs: These are the kanji of Warhammer, one you've memorized is your best friend, one you haven't is an annoying lookup problem. I think having no USRs creates problems when subtle differences creep into something you've reprinted a dozen times under different names, and they all need errataing if one of them does. I personally found 30K's USRs really hurt the accessibility of that game, making it very hard to eyeball what a unit or weapon actually does unless you've memorised a ton of the things, especially with them spread over multiple books. A small, canonical set in the corebook would be perfect.
"Not Colour Scheme Specific": I don't know what this really means until we see how Space Wolves, Dark Angels, Blood Angels and Deathwatch work. I think in the vast majority of cases any time you weren't playing a well known Marine chapter you were effectively playing "not colour scheme specific" anyway. I would like there to be less incentive to paint a Marine in successor chapter colours as opposed to a First Founding, since in the latter case you're handcuffing yourself mechanically and you don't really get anything out of it. I have Marines in 5+ chapter colours but for most of 9th I've been playing them as part of the same Crusade, fluffwise they know each other and fight in the same theatre, so why not make it easier for me to put them on the table together without giving me the ability to cherry pick abilities from across the range (multiclassing is the devil but I'll do it every time if you let me
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Datasheet Changes: Seem good. Like the AOS/KT style hit chance per weapon. OC is a good stat.
All my Codexes going in the bin: I bought a lot of Codexes and for the most part I really like them; since the second 8th edition Marines Codex I've enjoyed how much they make the armies feel like themselves with mechanics that get deep into their theme. They are also the big drivers of complexity and 9th's periodic balance nightmares (9's had a lot of healthy metas but also a shocking number of Voidweavers). Anything bad you've ever thought or heard about 9th probably came from a Codex. So...bye.
Still big questions about the core rules. Terrain rules are crucial. I think 9th's are the best I've seen in Warhammer but they could be better (and tbh I just really like Obscuring, but it's still hard to set up a table that uses it really well. Lots of TTS tables are *garbage*). The Fight phase is one of the biggest drivers of complexity (and bad feelings for the unprepared) outside the Codexes so does it get a big revision? Overall 9th's core rules have done a good job, will they mostly stay in place?
Very excited to play!
@MiJ - I think the last time we talked about this we established the Legends rules for your Corsairs are surprisingly good and useable in 9th. I don't know if that's guaranteed in 10th; maybe you should take them out for a last hurrah in 9th!